Field of the Invention
The present invention relates to an image forming system including an image forming apparatus that forms an image by transferring toner images onto a paper sheet, and to the image forming apparatus.
Description of the Related Art
Electrophotographic image forming apparatuses have been known in the form of printers, copying machines, and the like. In such an image forming apparatus, static charge might be generated to electrically charge a paper sheet at the time of image formation or conveyance of the paper sheet. When electrically-charged paper sheets are ejected and stacked outside of the apparatus, defective stacking might occur due to repulsion force or absorption force generated between the paper sheets.
In an image forming apparatus, a latent image formed on a photosensitive drum is visualized through development with toner. Primary transfer is performed to transfer the visualized toner image from the photosensitive drum onto a transfer belt, and secondary transfer is performed to transfer the toner image from the transfer belt onto a paper sheet. In this manner, an image is formed on the paper sheet.
So as to transfer a toner image onto a paper sheet, a secondary transfer unit applies a positive voltage to the paper sheet from its back surface, for example. As a result, the paper sheet that has passed through the secondary transfer unit is negatively-charged on the image formation surface onto which toner is transferred, and is positively charged on its back surface.
FIGS. 13A to 13C are diagrams for explaining an example of paper sheet charging. An example case where two-side printing is performed on a paper sheet P is now described. First, as shown in FIG. 13A, when a toner image is transferred onto one surface P10, positive charge E11 is applied to the other surface P11, which is the surface on the opposite side from the image formation surface, and negative charge E12 is applied to the toner layer P10a transferred onto the one surface P10. The positive charge E11 moves toward the negative charge E12 in the paper sheet P.
As shown in FIG. 13B, when the paper sheet P is reversed so that the toner image is transferred onto the other surface P11, positive charge E21 is applied to the toner layer P10a on the one surface P10, and negative charge E22 is applied to the toner layer P11a transferred onto the other surface P11. If this situation is left as it is, mutual attraction occurs between the positive charge E21 and the negative charge E12, between the negative charge E12 and the positive charge E11, and between the positive charge E11 and the negative charge E22. As a result, the one surface P10 and the other surface P11 of the paper sheet P are charged with the opposite polarities from each other.
Further, as shown in FIG. 13C, when paper sheets P are stacked, the positive charge E21 on a toner layer P10a faces the negative charge E22 on a toner layer P11a, with a gap G being interposed in between. Because of the opposite polarities, the negative charge E22 gradually moves toward the surface, and a high-density charged portion is formed on the surface side. With this, mutual attraction force is generated between the stacked paper sheets P, and the paper sheets P are bonded to one another.
To counter this problem, there is a technology that involves a charging unit that electrically charges paper sheets prior to stacking. According to this technology, a correction voltage in accordance with the potential of a paper sheet prior to stacking is applied to the charging unit, so that the paper sheet is electrically charged (see JP 2009-001418 A and JP 2013-227088 A, for example).
The unit for sensing the potential of a paper sheet can acquire the potential of only part of the paper sheet. Therefore, even if the correction voltage in accordance with the potential of a paper sheet prior to stacking is applied to the charging unit, appropriate charging cannot be performed on all the surfaces of the paper sheet.